Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Costs (Benefits) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 12	$ 14	$ 33	$ 40
Interest cost	27	25	82	75
Expected return on plan assets	(36)	(36)	(107)	(109)
Amortization of prior service cost (benefit)	1		2	1
Amortization of net actuarial loss	12	16	34	50
Net periodic benefit cost (benefit)	16	19	44	57
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	2	5	6
Interest cost	7	6	20	22
Expected return on plan assets	(6)	(5)	(18)	(15)
Amortization of prior service cost (benefit)	(2)	(4)	(9)	(6)
Amortization of net actuarial loss		2	2	10
Net periodic benefit cost (benefit)		$ 1		$ 17